SIGNAL CHAIN: Tom Anderson Cobra Special-S with 3 mini humbuckers cable to the unit by a Mogami Platinum 12' and from the unit with an Alleva-Copollo 20'.

The Twin Tube Blue is the third in a series of Twin Tube stompboxes manufactured by Seymour Duncan and Co., best known for their full line of excellent pickups. Each in the series uses a different subminiature triode tube; the “Blue” uses the 6111 military spec tube, hence the designation SFX-11. I’m sitting at my computer pondering the 3 lb. “Blue” and wondering what might the other two units do that this one doesn’t, and vice versa. Maybe it’s the old “collect all three and trade ‘em with your friends” thing… or not. All I can say is this unit does a lot and does it well.

On the outside, the unit is painted a very pretty metallic blue with well-done graphics and labels in contrasting colors. The bottom cover is black with a large corrugated rubber “slip strip” and slotted vent openings for cooling on each side. Four screws hold the beefy steel top and bottom together. The left hand side has a status light at the top to show that the unit is engaged. When the light is out, the unit is in true-bypass mode. Below the light is the master Tone section with separate Bass and Treble controls, affecting both Rhythm and Lead channels equally (but of course not affecting the bypass signal). Below that is the logo, and at the lower edge of the chassis is the On/Bypass switch, brilliantly located so that it may be actuated easily with a toe (preferably in a shoe).

The right side of the top has the Rhythm channel, with separate Volume and Gain knobs. Below that is the Lead channel, with separate Volume and Gain controls as well. Below that, similarly placed to the bypass switch, is the Channel Select switch, which allows either the rhythm channel or the lead channel to enter the signal path, but not both. The rear of the blue cover has a 1/4" input for the instrument and a like output into the amp (or other device). There is also an input for the 16V 560mA AC power supply from a wall wart—the only way of powering this unit.

Removing the bottom cover reveals a very neat layout and tidy soldering. The mounting of some of the heavier components is reinforced by neatly placed dabs of flexible glue. There are some beefy capacitors in there that could definitely knock your socks off, so don’t be trying this at home unless you are sure you know what you’re doing. The 6111s have wires extending from their bases (instead of pins) and the wires are nicely insulated from each other and neatly soldered to tabs. A skilled tech should be able to change tubes in about 15 minutes, should that ever become necessary. The tubes are held to the board horizontally with a removable metal clamp and cushioned somewhat by silicon or Teflon tabs on the clamp and the board. All components both inside and out, except the footswitches, are mounted to a fiberglass printed circuit board. Components appear to be of good quality. The pots and switches are labeled “Alpha,” a reliable brand.

Time to light this baby up!
First, I tested the bypass by A/B’ing the unit in bypass mode with a direct connection and heard no change. I then read the very well-written instruction manual and set the unit up using my Clark Beaufort 2x10" and Mogami Platinum and Alleva-Coppolo cables (both very neutral sounding).

At middle, but appropriate, gig volume levels, the rhythm channel sounded very transparent and easily duplicated the bypass sound when dialing in less gain and more volume on the channel, with the master Tones set for Bass at 11 o’clock and Treble at 2. I could do the same with the lead channel, but had to add bass and back off the treble. This indicated that the rhythm channel used a different voicing than the lead channel. Indeed, further tweaking and listening showed the Lead channel with more top and a bit less bottom than the Rhythm channel. This makes sense if you plan to use the unit in the standard way, since you’d want the lead to come forward in the mix without adding mud to the overall sound of the band.

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